1. Field of the Invention
The present invention relates to microphones, and, more particularly, to microphones that may be included in audio headsets, such as headsets that may be used within an aircraft during flight.
2. Description of the Related Art
Traditionally, microphones are included in wired headsets, such as headsets that are used within aircrafts by pilots, passengers, and others inside the plane. Such headsets are also used by football coaches to communicate instructions and plays during the course of the football game. Both airplanes and football games are generally noisy environments, and the background noise makes it difficult for the listener to clearly hear what the wearer of the headset is saying. Of course, such background noise is also a problem for hand microphones that are not used in headsets. For instance, a speaker, singer or musician may use a hand microphone when addressing a crowd of thousands of people, and often such a large crowd creates a lot of background noise.
Microphones systems utilizing digital signal processing techniques can remove noise and suppress unwanted background sounds that a traditional microphone otherwise would suffer. These microphones can cancel undesired background noise up to 20-24 dB. These noise suppressions result in cleaner audio signals at the pre-amplifier or far end listener in the case of communication microphones. However, the critical communication headsets (including microphone modules) demand that the microphone be activated instantaneously to in order to communicate voice. This requirement arises due to mission-critical usage scenarios of these microphone modules, such as in military situations, aircrafts, and many critical and emergency facilities.
Digital noise canceller microphones include digital signal processors (DSPs) and complex electronics, and thus such microphones can suffer from reliability issues due to the failure of partial systems. These failures are mainly due to the large number of failure modes associated with the many components and complex signal processing algorithms used in these modules. Analog microphone modules, in comparison, are relatively simple and reliable. Thus, these reliability issues present new challenges in the design of microphones that use digital signal processing techniques.
Digital microphones have other disadvantages in comparison to analog microphones. Specifically, digital noise cancellation microphones typically take a longer time to turn on than analog microphones due to the boot loading time of firmware and power-on self verification times of the microprocessor or digital signal processor chips. In addition, digital noise canceling microphones may consume more power than their analog counterparts.
Communication microphones are used mainly for two-way communications in conjunction with an intercom system. In traditional communication systems, speech signals are picked up by an analog microphone and passed into an intercom system. The intercom interface can be a stand-alone unit or may take the form of a belt pack or a telephone interface.
Microphones pick up speech as well as noise signals within the frequency range of the microphone. This noise gets added to the speech or other wanted signals and degrades the quality of the audio signal. With digital signal processing techniques, however, noise components can be removed, leaving only the wanted speech signal unchanged. These techniques are needed when the background noise levels are relatively high. In typical digital microphone noise cancellation systems, two microphones are used. One microphone is used as a noise microphone, and the other microphone is used as a speech microphone.
With the noise source being a relatively far-fielded signal, both the noise microphone and the speech microphone experience a similar transfer function in response to the noise sources. In contrast, the speech source is physically closer to the speech microphone, and thus the speech microphone and noise microphone experience different transfer functions in response to the speech sources. Thus, the two microphones may pick up the noise signal source similarly, but the speech signal source being closer to one microphone can result in different speech transfer functions in the two microphones.
Digital dual microphone noise cancellation has been used in many communication systems based on the principal of identifying the two microphones' transfer functions for the speaker source and for the noise source. The microphone module may also use spectral subtraction techniques to remove common noise components. The noise reduction is achieved by suppressing the effect of noise on the magnitude spectrum only. The subtraction process is performed in power terms or true magnitude terms depending upon the ambient noise source. The important point is that phase terms are ignored for practical reasons in that phase has limited influence on the result. A built-in voice activity detector (VAD) is used to distinguish the speech segments from the noise segments. This distinction is used for proper tuning of the subtracting stage.
Cross-fading algorithms are a known technique to mix two or more audio signals without audible pops, glitches or hard audible clicks being created as artifacts. In simplest form, cross-fading involves the use of two volume controls. One volume control is used to reduce the volume of one audio signal, while the other volume control is used to increase the volume of the other audio signal, with the total volume of the two signals remaining constant.
The boot up time of the digital microphone system may take up to about one full second, and, in critical communication microphones, this delay can result in loss of valuable information. Modern DSPs and microcontrollers use firmware to perform algorithm operations. The firmware gets loaded from a memory module that is connected to the system via a bus. This loading of an instruction sequence to the DSP or microcontroller takes a considerable length of time. In addition, these DSPs and microcontrollers take time to check power during self diagnostic operations and startup operations. Even after the system has started operating, additional time is required for the input and output buffers to be filled in order for the algorithms to effectively remove noise from the input signals.
What is needed in the art is a microphone system that avoids the above-mentioned problems and disadvantages.